In the following particular reference to combustion devices that are part of a gas turbine is made; it is anyhow clear that the method can also be implemented in combustion devices for different applications. Thus, before the combustion device a compressor and after the combustion device a turbine are typically provided.
Combustion devices are known to include a body with a fuel supply for either a liquid fuel (for example oil) or a gaseous fuel (for example natural gas) and an oxidizer supply (usually air).
During operation, the fuel and the oxidizer react within the combustion device and generate high pressure and temperature flue gases that are expanded in a turbine.
During transient operation, such as for example when the gas turbine is started up, switched off, during fuel switch over or also during other transient operations, problems can occur.
In fact, during transient operations pressure waves can generate within the combustion device.
FIG. 1 shows an example of a possible circumferential pressure wave (it can be a static or a rotating pressure wave). FIG. 1 shows the pressure P as a function of the angular position φ over the combustion device at a period in time t=t0 (solid line) and t=t1 (dashed line). From this figure it is apparent that an injector located at a position φ1:
at the period in time t=t0 faces an environment at a low pressure P1; this promotes fuel supply through the injector; and
at the period in time t=t1 faces an environment at a high pressure P2; this hinders fuel supply through the injector.
Likewise, FIG. 2 shows an example of a possible axial pressure wave. FIG. 2 shows the pressure P as a function of the axial position x (L indicates the combustion device length) at a period in time t=t0 (solid line) and t=t1 (dashed line).
Also in this case, an injector will face a combustion device having a pressure that fluctuates with time; as explained above, this fluctuating pressure adversely influences fuel injection.
FIG. 3 shows the effect of the fluctuating pressure within the combustion device on the fuel injection. In particular FIG. 3 shows an example in which the fuel mass flow is reduced; this could be an example of a switch off, nevertheless the same conditions are also present at the beginning of a start up or at the beginning and end of a switch over and in general each time the fuel mass flow supplied decreases and falls below a given mass flow.
FIG. 3 shows the fuel mass flow M injected through an injector as a function of time t. From FIG. 3 at least the following phases can be recognized:
before t=t3: steady operation with substantially constant fuel mass flow through the injector (curve 1),
between t=t3 and t=t4 (the fuel mass flow stays above a critical fuel mass flow Mc): the amount of fuel injected decreases, but the fluctuating pressure within the combustion device does not noticeably affect fuel injection (curve 2),
after t=t4 (i.e. when the fuel mass flow falls below the critical fuel mass flow Mc): in these conditions, since the amount of fuel is low, the fluctuating pressure within the combustion device alternatively promotes and hinders fuel injection, causing a fluctuating fuel injection. In particular in FIG. 3, curve 2 shows a theoretical run of the reducing fuel mass flow and curve 3 an example of a possible real run of the reducing fuel mass flow.
Fluctuating fuel supply into the combustion device generates large combustion pulsations.
Combustion pulsations, largely mechanically and thermally, stress the combustion device and the turbine downstream of it, therefore they must be counteracted.